Advanced composite materials are being used increasingly in the aerospace and other industries. The materials include a number of layers or plies which have reinforcing fibers in various woven patterns or unidirectional orientations, such as 0.degree., 90.degree., and .+-.45.degree.. In many of the materials, the reinforcement is provided by graphite fibers Graphite fibers are very hard and make the composite materials abrasive This results in very high wear rates of cutters used to trim the edges of workpieces made from such materials. In order to counteract the problem of excessive wear, polycrystalline diamond (PCD) cutting edges are sometimes used to machine composite material parts having graphite reinforcing fibers. The PCD cutting edges have wear resistance superior to more conventional cutting edge materials.
In spite of their superior wear resistance, the use of PCD cutting elements has presented a number of problems. PCD elements are relatively expensive and are only available in the form of flat wafers Since PCD material is very brittle, the flat wafers cannot be bent even a small amount. Therefore, they cannot be used to construct a cutter with a helical cutting blade.
Because of the limitation of PCD cutting blades to a flat configuration, currently available PCD cutters are made with the cutting blade and cutting edge parallel to the longitudinal axis of the cutter, as illustrated in FIGS. 1 and 2. This cutting blade configuration has resulted in a number of problems which have greatly decreased the practical usefulness of cutters with PCD cutting elements The main problems that have been experienced are excessively high cutting forces and insufficient quality of the surface finish of the workpiece. The high cutting forces are caused by the fact that every point along the full length of the PCD cutting edge hits the workpiece being trimmed at the same time. When the full length of the cutting element hits the workpiece, high impact forces are created. These forces cause excessive vibrations in the workpiece and in the spindle of the cutter. In addition, the high cutting forces generally prohibit the use of known types of cutters with PCD cutting elements in hand-held routers.
The relatively poor surface finish obtainable by use of known types of cutters with PCD cutting blades is primarily a result of the fact that all of the cutting forces are coplanar with the layers of the composite material laminate. Two main types of degrading phenomena are observed in workpieces machined with known cutters. One of these is a commonly observed phenomenon in which some of the plies of the workpiece are cut deeper than others. Although the finished edge may appear to be smooth to the naked eye, under a microscope the differences in the plies are clearly observed These differences give the workpiece a poor surface finish relative to the standards of the aerospace industry. The phenomenon is illustrated in greatly exaggerated form in FIG. 4. Therein, it can be seen that the plies of the workpiece with 0.degree. unidirectional fibers, i.e. fibers that run in the same direction as the direction of the cutter feed, tend to cut deeper than the other plies. The problem of differential depth of cut is especially pronounced in relation to workpieces having thermoplastic resin matrices.
FIG. 3 illustrates another commonly observed phenomenon of the surface finish obtained using known types of PCD cutters. This phenomenon occurs primarily in plies having -45.degree. fibers. The phenomenon is also observed, to a lesser degree, in +45.degree. and 90.degree. plies. The fibers in such layers tend to be bent and bunched up by the cutter as it moves across the workpiece. Voids are formed between the bunches of fibers. These voids are clearly visible under microscopic examination. The resulting uneven and discontinuous nature of the machined edge of the layers significantly degrades the surface finish of the workpiece.